(a) Field of the invention
The present invention relates to novel tetrazonium salt compounds, novel disazo compounds and the method of producing said compounds, and electrophotographic elements containing these disazo compounds and the like, in particular relates to electrophotographic elements provided with photosensitive layers containing said disazo compounds as materials that generate charge carriers when exposed to light (which will be called charge carrier generating materials hereinafter), preferably multilayer type electrophotographic elements comprising layers containing said charge carrier generating materials (which will be called charge carrier generating layers hereinafter) and layers containing materials which receive the charge carriers generated in said charge carrier generating layers and transfer them (which will be called charge transfer materials hereinafter) respectively.
(b) Description of the prior art
As the conventional electrophotographic elements, there can be enumerated inorganic and organic ones. The inorganic system electrophotographic elements include those using selenium and its alloys and those prepared by dispersing dye-sensitized zinc oxide in binder resins, while as the organic system electrophotographic elements, there can be typically enumerated those using a charge transfer complex of 2,4,7-trinitro-9-fluorenone (which will be called TNF hereinafter) and poly-N-vinylcarbazole (which will be called PVK) and the like. But, it is also fact that these electrophotographic elements have various advantages, while having various disadvantages. For instance, the selenium electrophotographic elements which have presently been used universally are defective in that the manufacturing conditions are strict, the manufacturing cost is expensive, it is difficult to process them into belt-like form due to the absence of flexibility, attention must be paid in handling them because they are highly sensitive to heat and mechanical impact. Referring to zinc oxide elements, the manufacturing cost is low because they can be manufactured by applying cheap zinc oxides onto substrates, but said electrophotographic elements are defective mechanically in that they are generally inferior in sensitivity, surface-smoothness, solidity, tensile strength, friction resistance and the like and involve various problems to be solved in respect of durability and the like as the elements used repeatedly in copying plain papers. The electrophotographic elements using charge transfer complexes of TNF and PVK are so inferior in sensitivity that they are not suitable for the elements for use in high-speed copying machines.
Of late years, a wide range of studies have been carried out in order to eliminate the shortcomings inherent in these electrophotographic elements. In particular, various organic electrophotographic elements have been proposed for that purpose. Among them, multilayer type elements are attracting public attention as electrophotographic elements for use in plain paper copying machines due to their high sensitivity and stable chargeability as compared with usual organic electrophotographic elements, said multilayer type electrophotographic element comprising an electrically conductive substrate, a charge carrier generating layer formed by depositing on said electrically conductive substrate a thin film of organic pigment; and a charge transfer layer formed on said charge carrier generating layer and consisting essentially of a charge transfer material. And, some of them are put to practical use.
As the conventional multilayer type electrophotographic elements of this sort, there are known:
(1) the multilayer type electrophotographic element using, as the charge carrier generating layer, a thin layer formed by vacuum-vapordepositing a perylene derivative and incorporating an oxadiazole derivative in the charge transfer layer (which see U.S. Pat. No. 3871882),
(2) the multilayer type electrophotographic element using, as the charge carrier generating layer, a thin layer formed by coating on organic amine solution of Chloro Dian Blue and incorporating a hydrazone compound in the charge transfer layer (which see Japanese Patent Publication No. 42380/1980),
(3) the multilayer type electrophotographic element using, as the charge carrier generating layer, a thin layer formed by coating an organic solvent dispersion of distyrylbenzene type disazo compound and incorporating a hydrazone compound in the charge transfer layer (which see Japanese Laid Open Patent Application No. 84943/1980).
However, the fact is that even in the multilayer type electrophotographic elements of this sort, the conventional ones have a number of advantages as well as various disadvantages.
That is, the electrophotographic element using the perylene and oxadiazole derivatives disclosed in the preceding (1) is disadvantageous in that the cost of production is raised because the charge carrier generating layer is formed by vacuum vapor-deposition.
The electrophotographic element using the Chloro Dian Blue and hydrazone compound disclosed in the preceding (2) involves disadvantages in the preparation because there is necessity of using a hard-to-handle organic amine (for instance, ethylene-diamine) as a coating solvent for the formation of the charge carrier generating layer. Further, this electrophotographic element is inferior in the reproductivity of red images from the original because its visible light wavelengths cover the range of about 450-660 nm. Due to this, it is necessary to employ a filter to cut a red light when this element is actually set in the copying machine, thereby exerting an unprofitable influence upon the copying machine design.
The electrophotographic element using the distyrylbenzene type disazo compound and hydrazone compound disclosed in the preceding (3) is very profitable in the preparation because the charge carrier generating layer can be formed readily by coating a dispersion of disazo compound, but is defective, like the electrophotographic element disclosed in the preceding (2), in that the reproductivity of red images from the original is inferior because its sensitive light wavelengths cover the range of about 450-700 nm.
As the disazo compounds used in the multilayer type electrophotographic elements, there are also known, for instance, the benzidine type disazo compound disclosed in Japanese Laid Open Patent Application Nos. 37543/1972 and 55643/1977, the stilbene type disazo compound disclosed in Japanese Laid Open Patent Application No. 8832/1977. However, the multilayer type electrophotographic elements using these conventional disazo compounds were generally low in sensitivity and deteriorated in the reproductivity of red images from the original because their sensitive light wavelengths cover the range of about 450-700 nm. Accordingly, these electrophotographic elements were unprofitable in the copying machine design as stated previously, because these elements had to employ a filter to cut a red light when they are actually set in the copying machines.